Today sensor chips or beads are commonly mounted in standard housings, such as e.g. TO18, which is commonly used for housing transistors or other microelectronic devices. Another commonly used housing type is in the style first used by Figaro Engineering Inc. for its TGS 821—Special Sensor for Hydrogen Gas, which housing type is today used by many manufacturers of sensors.
Common for all sensor housings are that they are made to mechanically protect the sensing elements thereof against e.g. mechanical impact, dust and thermal shock.
In gas sensors gas may reach the sensor through directing a flow of sample gas onto an active surface of the sensing element. Although such a procedure ensures rapid establishment of the correct gas concentration on the active surface of the sensor, there is a significant risk that changes in flow or temperature may give rise to false readings.
Directing a gas flow onto the active surface also leads to an increased risk of particle contamination. It is therefore common that the sensing element is arranged to communicate with the ambient gas through diffusion only. A common way of doing this is to arrange a fine filter, such as a sintered disk or PTFE membrane in front of the sensing element. Such a fine filter is also commonly called diffusion membrane.
The diffusion principle is very robust but still suffers from a few drawbacks, such as slower establishment of the correct gas mixture on the active surface.
In one example application, manual leak detection, the sensor is often brought into close vicinity of the suspected leakage point or simply swept over a surface to be checked for leakage. It is evident that for such a use a high speed of response is beneficial.
When moving the sensor to a leak point there is also imminent risk of mechanical impact and contamination by dirt such as e.g. dust, oil and grease.
The housings of today commonly available sensors are typically not well suited to withstand such environments and leak detection probes are therefore normally fitted with at least one extra barrier in the probe tip for protecting the actual sensor housing. Such an extra protection barrier also adds to the delay in response of the detector.
Today commercially available sensors usually reside in housings which are typically equipped with a number of contact pins. A leak detection probe thus typically has a matching contact socket into which the contact pins of the sensor housing are inserted.
These today commercially available sensors are seldom suitable for application at the tip of a probe without any additional protection. Some sort of protective “cap” is therefore normally snapped or screwed on top of the sensor.
In addition to increasing the total distance, such a set up results in, at least, two filter barriers with a closed volume between them. The arrangement further reduces the diffusion speed by dividing the concentration gradient in two smaller gradients, each communicating with a volume closer to the active surface.
Usually, each of the individual filter barriers will need to be self supporting and therefore require a certain thickness. Thus, provision of one or more extra filter barriers usually renders an increase of thickness exceeding a mere aggregation of thicknesses.